1. Field of the Invention
The present invention relates to a lubricant composition, more precisely, to that favorable for hydraulic oil to be used for power transmission in construction equipment, machine tools, etc.
2. Description of the Related Art
Regarding its history, hydraulic oil has developed from zinc-containing oil into long-life zinc-containing oil and now into non-zinc oil. Non-zinc oil has good oxidation resistance, sludge resistance and wear resistance but is defective in that its extreme pressure resistance and fatigue life are inferior in some degree to those of zinc-containing oil. Therefore desired is non-zinc hydraulic oil which has its own advantages of good oxidation resistance, sludge resistance and wear resistance and further has improved extreme pressure resistance and prolonged fatigue life. JP-A 9-111277, 2000-303086 and 2000-169871 disclose typical examples of non-zinc hydraulic oil that contains a phosphate such as typically tricresyl phosphate. However, the phosphate is defective in that it often causes seizure and serious wearing of hydraulic tools that are driven under high pressure especially under 30 MPa or higher.
From the viewpoint noted above, we, the inventors have made the present invention. The invention is to provide a lubricant composition favorable for non-zinc hydraulic oil, which has good oxidation resistance, sludge resistance and wear resistance and further has improved extreme pressure resistance and prolonged fatigue life.
We, the present inventors have assiduously studied and, as a result, have found that the object of the invention can be effectively attained by using additives of a phosphate, an amine salt of an acid phosphate, and a sulfur-containing extreme-pressure agent. On the basis of this finding, we have completed the present invention.
Specifically, the subject matter of the invention includes the following:
1. A lubricant composition prepared by adding, to abase oil, (a) from 0.01 to 5% by mass, based on the composition, of a phosphate, (b) from 0.005 to 1% by mass of an amine salt of an acid phosphate and (c) from 0.01 to 1% by mass of a sulfur-containing extreme-pressure agent.
2. The lubricant composition of above 1, wherein the sulfur-containing extreme-pressure agent is a thiadiazole compound.
3. The lubricant composition of above 1 or 2, wherein the phosphate is tricresyl phosphate.
4. The lubricant composition of any of above 1 to 3, wherein the amine salt of an acid phosphate is dodecylamine salt of acid mono(di)-methyl phosphate.
5. The lubricant composition of any of above 1 to 4, which is used for hydraulic oil.